This research proposal seeks to delineate the pathophysiologic mechanisms involved in the hypoglycemia frequently seen in small for gestational age (SGA) newborns. It seeks to do this by a prospective study during the first six hours following birth of the metabolic sequence involved in the adaptation to extrauterine fasting. Particular emphasis will be placed on capacity for gluconeogenesis, availability of substrate for gluconeogenesis, availability of alternate fuels and hormonal milieu. This studies will compare the patterns observed in SGA infants who maintain normoglycemia, SGA infants who become hypoglycemic, premature infants who are appropriate for gestational age (AGA) who maintain normoglycemia, AGA prematures who develop hypoglycemia, and full term normoglycemic AGA newborns. In addition, an animal model for the SGA newborn will be tested by studying "runted" guinea pigs. We will study the ability of these newborns to withstand fasting. In tissues prepared from these animals, we will carry out in vitro studies of gluconeogenesis and ketogenesis by isolated liver cells, and glucose uptake and ketone utilization by an isolated muscle preparation.